This invention relates generally to sensing the level of fluids in a reservoir. More particularly, the present invention relates to sensing fluid levels in reservoirs that are part of fluid dispensing apparatus used in the manufacture of substrates or supports having bound to the surfaces thereof a plurality of chemical compounds, such as biopolymers.
Many devices are known that operate to monitor fluid level in vessels or reservoirs such as tanks, storage containers and the like. Many devices operate by means of an electromechanical switch that is activated by a float. The float rides or floats on the surface of the fluid. When the fluid reaches a given level, a switch coupled to the float is operated.
There are other systems that utilize electrical or electronic devices to detect the level of fluid in a vessel or reservoir. For example, fluid level controls for containers such as boilers are known. Conductance probe controls detect a level of a conductive media, such as a fluid, by passing an electrical current through a probe, into the conductive media and then back to a controller. The probe is at least partially immersed in the fluid when the fluid is at certain levels and not in contact with the fluid when the fluid is at lower levels. The controller measures the received current level to determine whether the probe is in contact with the media, i.e., whether a fluid is at or above a certain level. As fluid engulfs the electrode, electrical conductivity between the electrode and the reservoir body is increased. This increase is detected electronically and a digital signal is produced indicating the presence or absence of fluid.
Such devices conventionally utilize AC currents, as applying a constant DC current to a probe exposed to a conductive fluid can undesirably promote probe deterioration. However, a drawback of using AC is that it requires the inclusion of support circuitry when also using a conventional microcontroller. This increases the cost and complexity of such a device. Three examples of commercially available sensors using AC currents can be found at these websites:    http://www.measure.com/sensors/sensor-soil.htm (see “Introduction”),    http://www.deltacnt.com/512.htm (see “Features” and “Operating Principle”),    http://www.gemssensors.com/operatingCLS1200.htm (See “Operating Principle” and comparison chart.)
A device for sensing a level of a fluid is disclosed in U.S. patent application Publication 2002/0189346 A1 (Thomson) published Dec. 19, 2002, filed Jun. 6, 2001. The device of Thomson includes a microcontroller, a probe coupled to the microcontroller, and a plurality of instructions for use by the microcontroller to apply a given DC potential to the probe for a first period of time, measure a voltage associated with the level of the fluid to be sensed after the first period of time, and apply a reference DC potential to the probe for a second period of time.
There remains a need for a device for sensing fluid levels in a fluid storage vessel. The device should avoid AC sources and promote safety by limiting probe excitation to safe levels. The device should be compact and comprise a minimum number of components so that they may be employed in compact locations. It is also desirable to have methods and apparatus for avoiding damage to the sensing probe. The methods and apparatus should be particularly applicable to droplet dispensing devices for preparing a plurality of chemical compounds on a surface of a substrate to form a plurality of features such as, for example, an array of features on the substrate.